Dzhokhar Tsarnaev is an admitted accomplice in the 2013 Boston Marathon bombing; a federal court will soon decide on his punishment. His attorneys, hoping for leniency, have argued, plausibly, that he was an immature young man whose overbearing older brother coaxed him into a terrorist act about which he was ambivalent.

Federal prosecutors, meanwhile, have argued that Tsarnaev was a committed terrorist whose radicalization was largely the result of his own self-cultivated ideological beliefs. Copies of Al Qaeda’s Inspire magazine, as well as provocative speeches by extremist Muslim clerics pulled from Tsarnaev’s laptops, have been marshaled as evidence that he had in fact been largely radicalized on his own through the influence of materials he procured on the internet.

The question of whether Dzhokhar was driven to terrorism by the influence of his strong-willed older brother, Tamerlan, or by charismatic online “jihadist” preachers such as Anwar al-Awlaki, is one upon which his life may now depend. His lawyers have argued that whatever role these materials may have played — or whether he even read or listened to them at all — is immaterial compared with the influence his older brother had in goading him into joining the 2013 marathon bombing. Prosecutors have countered this argument by soliciting high-profile terrorism analysts to make the case that Dzhokhar’s own online activities could have been enough to push him toward terrorism.

In testimony given last week, prosecution witness Matthew Levitt, a fellow at the Washington Institute for Near East Policy, attested to the existence of an online “global jihad movement” capable of indirectly mobilizing lone wolves like Tsarnaev. Repeatedly invoking Thomas Friedman’s aphorism that “the world has been flattened” by communications technology, Levitt suggested that the influence of Tamerlan was superfluous, since Tsarnaev “could just as easily been [radicalized] by someone online.” The testimony seemed to lay the groundwork for seeking the death penalty against Tsarnaev in a later phase of the trial.

The defense should be able to mount a strong rebuttal when it comes time to settle on Tsarnaev’s punishment.

For example, one data point supporting the defense’s version of events is a 2013 study published by the think tank Rand Corp., which found that the internet alone is an insufficient channel for an individual to become radicalized. “There is evidence to suggest that in the large majority of cases, radicalization does not occur solely through the internet, but instead involves offline contact,” the study says, going on to add that “even those witnesses who attributed a significant role to the internet tended to support [the] conclusion that some element of face-to-face contact was generally essential to radicalization taking place.”

And while prosecutors have attempted to characterize Dzhokhar as a “holy warrior,” much of what is known about his life leading up the bombing — his reported pot-smoking and partying — flies in the face of the idea that he was an ideologically committed terrorist in the service of militant Islam. Indeed, his own behavior seems to contradict formal models of radicalization.

A model developed by terrorism researcher Chris Heffelfinger in his book Radical Islam in America suggests that individuals who are ideologically committed radicals of any type tend to move through four phases on the path to violence, beginning with an introduction to extremist ideas, continuing through to immersion and solidification of an extremist mindset, followed by expressions of frustration with the perceived inaction of fellow group members, and finally, an act of violence.

Under this model, a terrorist acting in the name of Islam would typically embrace austere, strict approaches to the religion, like Salafism, as part of this conveyor belt toward violence. Dzhokhar, in stark contrast, continued to be a recreational drug user whose lifestyle did not comport at all with that of others who moved along these various stages toward ideologically driven violence.

Lending further credence to the idea that Tsarnaev was swayed by his brother is a 2014 study published in the journal Psychological Medicine, which found that sibling relationships are often a major driver of violent criminality, specifically when the elder sibling is the one pursuing such criminality. According to the study, “Because older siblings often exert more influence on siblings than younger, the risk for violent criminal behavior should be greater when the older sibling has violent criminal behavior.”

Dzhokhar’s older brother, Tamerlan, is widely believed to have been violent, unstable and overbearing, and is also more likely to have exhibited traditional patterns of behavior associated with violent radicalization — so much so that he landed on the radar of the Russian spy agency FSB as a potential risk for extremist violence. A 2013 profile of the brothers quoted a former wrestling teammate of Dzhokhar’s who suggested that even Dzhokhar’s interest in the sport originated with his older brother, saying, “He’s done these violent sports because his brother’s a boxer … He really loves his brother, looks up to him.”

Furthermore, the absence of Dzhokhar’s parents may have increased the amount of sway Tamerlan had over him. “Given his separation from his parents was longer than he wished, his brother would come to have more influence on him than might otherwise have been the case,” said clinical psychologist Alice LoCicero, author of Why Good Kids Become Deadly Terrorists, a study of homegrown radicalization in the context of the marathon bombing. LoCicero added that widespread perceptions of Dzhokhar’s laid-back attitude, attested to by his peers, may actually have contributed to his susceptibility to influence from a strong-willed older brother like Tamerlan.

The defense in Dzhokhar’s case rested yesterday, setting the stage for the trial to move from the guilt phase to the penalty phase. The question of whether Tsarnaev ultimately receives the death penalty will thus soon be settled, in large part based on whether the jury comes to see him as an ideologically driven terrorist, or an impressionable young man coaxed into action by a violent older sibling.